1. Field of the Invention
The present invention relates to a shut-off and pressure-relief device for controlling a supply of foam precursors into a foam mixing and dispensing means, and for annulling the pressure at the closed mouth of the dispensing means relieving the pressure in the supply line.
2. Description of the Prior Art
A polyurethane foam mixing and dispensing gun comprising a mixing chamber, a retractable or non-retractable shut-off rod, which can shut off the mixing chamber over the entire length, an actuating mechanism for moving the shut-off rod in both directions, and a grip, is widely known.
The mixing chamber consists of a cylindrical cavity into which a shut-off rod fits accurately, which rod may be moved alternately in opposite directions by means of an actuating mechanism which consists, for example, of a pneumatic, hydraulic or electrical cylinder actuator.
The foam precursors are conveyed separately towards the mixing chamber via supply lines, which are generally under a pressure of at least 6 to 12 bar.
In the most widely known systems, a certain pressure remains along the entire length of the supply line through which the foam precursors are conveyed. In order to be able to isolate the mixing chamber during shut-downs or interruptions, hand-actuated leakproof shut-off gate valves are provided on each side of the metering apparatus. These gate valves are intended to prevent the infiltration of foam precursors into the mixing chamber of the gun, when the latter is not in use.
The foam precursors are packed into steel cylinders under a nitrogen pressure of 6 to 12 kg/cm2 and are delivered by a system under a pressure which may reach 30 bar. The shut-off gate valves allow the lines or drums to be isolated, but these still remain without pressure, even after any interruption in the operation of the metering apparatus.
The foam precursor supply pipe portions, which are shut off at each end, by the aforementioned valve and the shut-off rod still remain under pressure preload, even after prolonged shut-downs. For this reason, minuscule quantities of the two components may react rapidly and form extremely fine sticky foam particles, which even when there are only traces of them, may block the shut-off rod in the mixing chamber.
In the event of the shut-off rod jamming, or of the metering gun becoming obstructed, the mixing chamber and shut-off rod have to be dismantled for cleaning or replacement.
In order to avoid the leakage of liquids, particularly of liquid foam-precursors, into a shut-off line, and overcome the drawbacks of inopportune obstructions, the invention proposes a device for shutting-off and releasing the pressure in a supply line, said device comprising a first and a second supply unit, each having a first input for inputting each time one of said precursors, and a first output connected to a respective supply line made of flexible material, said supply units each having a second input for supplying a driving power, said supply units further comprise a piston type pressure pump, having an expansion and a pressure chamber and a piston provided to be driven in a first direction into said chambers by means of said supplied driving power, said supply units also comprise a tube in connection with their first input and a one-way valve which outputs into a passage towards said first output and said pressure chamber, said one-way valve being provided for supplying said foam precursor to said pressure chamber, said piston being further provided to be driven in a second direction, opposite to said first, when said foam precursor is supplied to said pressure chamber, said expansion chamber having a third input for supplying a solvent, provided for rinsing said expansion chamber and said piston, said one-way valve being provided to remain closed during substantially a whole movement of said piston along said first direction and to remain open during substantially a whole movement of said piston along said second direction.
The supply of a solvent, via the third input, enables to undo the expansion chamber and the piston from particles of the substance which would remain stuck to them. In such a manner obstructions are avoided.
According to a first preferred embodiment, wherein said second and third input form a single input, and wherein said solvent also forms an hydraulic fluid for providing said driving power, said single input being connected to an hydraulic unit for storing said solvent. In this embodiment the driving power and the cleaning action are performed by the same fluid.
According to a second embodiment, said one-way valve comprises a rod, extending in said passage, said rod being provided to be actuated by said piston, in order to open said one-way valve, when said piston reaches a lower inversion point. The rod enables an easy and reliable command of the valve.